The present invention relates generally to MR systems and, more particularly, to a shim coil of a magnet assembly of an MR system that achieves a near-homogeneous magnetic field with reduced B0 field settling time.
It is generally known that when a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
During fabrication and construction of the magnet assembly for an MR assembly, manufacturing tolerances and deviations in material make-up of the magnet assembly result in an inhomogeneous B0 field being created by the magnet assembly absent shimming. As a result of the magnet manufacturing process, it is not uncommon for the magnet to produce a very inhomogeneous field ranging from several hundred parts per million (ppm) to several thousand ppm, and a non-accurate center magnetic field that is significantly out of range. The importance of these variations is glaringly apparent given that MR systems require an intense uniform magnetic field, typically less than 10 ppm of variations within a 40-50 cm spherical volume, but also an accurate center magnetic field value, typically less than 0.5% variation. Compounding the field inhomogeneity is that contributed by the patient itself.
Shimming is a common process that is used to remove inhomogeneities from the B0 field. Shimming is important for MR systems because the average B0 field strength must be within a certain window for the RF hardware of the system. A simplistic example of the effects of shimming is graphically shown in FIG. 1. As shown, a magnet assembly without shimming produces a magnet field represented by curve 2. The variations of the magnetic field are quite clear. As is widely known, these variations negatively affect data acquisition and reconstruction of an MR image. As such, it is desirable to generate a shim field, represented by curve 4, that counters or offsets the variations in the magnetic field. The combination of the shim field 4 with the magnetic field 2 yields, ideally, a homogeneous and uniform B0 field represented by curve 6.
The shimming process includes the precise placement of shim elements within the magnetic assembly such that numerous small magnetic fields are generated to offset variations in the B0 field. The shim elements include active shims such as shim coils or permanent magnets as well as passive shims such as iron pieces. Shim coils are common in superconducting magnet assemblies and their shimming may be controlled by regulating current thereto. Course adjustments in field homogeneity for superconducting magnets are usually made with superconducting shim coils located within the helium vessel. Fine adjustments are more commonly achieved through one or more room temperature (RT) shim coils connected to a high-stability multi-channel power supply. Adjustments to the RT shim coils cause a reaction in the main superconducting magnet and any supplementary superconducting coils as they attempt to conserve flux according to Lenz' law. Furthermore, in order to improve its quench robustness, the main coil (magnet) is often divided into multiple sections where each section is protected with its own dump resistor. The magnet sections initially react independently to the RT shim adjustment and the resulting unmatched currents cause a temporary flow of current through the dump resistors. The current flow subsequently decays from the resistors, resulting in an undesirable field settling effect.
Notwithstanding the undesirable impact on settling time, conventional shim coils are constructed without regard to the affects the shims have on the B0 field settling time. That is, the primary objective is to construct the shim coil to improve field homogeneity. As a result, it is not uncommon for a given shimmed magnet to have a field settling time on the order of minutes. This settling time necessarily increases scan time and negatively affects throughput. More specifically, once the MR scanner is powered, scanning cannot commence until after the B0 field has settled and observing several minutes for that field to settle can significantly increase scan time.
It would therefore be desirable to have shim coil that is constructed with the impact on field settling time considered.